1. Field of the Invention
Embodiments disclosed herein relate generally to composite materials used in cutting tools. In particular, embodiments disclosed herein relate to methods for forming composite materials used in cutting tools.
2. Background Art
Historically, there have been two types of drill bits used drilling earth formations, drag bits and roller cone bits. Roller cone bits include one or more roller cones rotatably mounted to the bit body. These roller cones have a plurality of cutting elements attached thereto that crush, gouge, and scrape rock at the bottom of a hole being drilled. Several types of roller cone drill bits are available for drilling wellbores through earth formations, including insert bits (e.g. tungsten carbide insert bit, TCI) and “milled tooth” bits. The bit bodies and roller cones of roller cone bits are conventionally made of steel. In a milled tooth bit, the cutting elements or teeth are steel and conventionally integrally formed with the cone. In an insert or TCI bit, the cutting elements or inserts are conventionally formed from tungsten carbide, and may optionally include a diamond enhanced tip thereon.
The term “drag bits” refers to those rotary drill bits with no moving elements. Drag bits are often used to drill a variety of rock formations. Drag bits include those having cutting elements or cutters attached to the bit body, which may be a steel bit body or a matrix bit body formed from a matrix material such as tungsten carbide surrounded by an binder material. The cutters may be formed having a substrate or support stud made of carbide, for example tungsten carbide, and an ultra hard cutting surface layer or “table” made of a polycrystalline diamond material or a polycrystalline boron nitride material deposited onto or otherwise bonded to the substrate at an interface surface.
Most cutting elements or inserts on roller cone bits are made of tungsten carbide, a hard material, interspersed with a binder component, preferably cobalt, which binds the tungsten carbide particles together. Conventional tungsten carbide composites are formed by subjecting green bodies of tungsten carbide particles and binder to a sintering process, typically vacuum sintering and/or hot isostatic pressing (HIP), such as that described in U.S. Pat. No. 4,684,405. The green body is heated to an elevated temperature in a controlled atmosphere to sinter the tungsten carbide particles together. HIP, as a sintering technique, has also enabled further densification of the sintered part and minimization of fracture initiating voids.
Cemented tungsten carbide composites, such as WC—Co, are well known for their mechanical properties of hardness, toughness and wear resistance, making the composites a popular material of choice for use in such industrial applications as mining and drilling where their mechanical properties are highly desired. Because of the desired properties, cemented tungsten carbide has been the dominant material used as cutting tools for machining, hard facing, wear inserts, and cutting inserts in rotary cone rock bits, and substrate bodies for drag bit shear cutters. The mechanical properties associated with cemented tungsten carbide and other cermets, especially the unique combination of hardness toughness and wear resistance, make these materials more desirable than either metals or ceramics alone.
Many factors affect the durability of a tungsten carbide composite in a particular application. These factors include the chemical composition and physical structure (size and shape) of the carbides, the chemical composition and microstructure of the matrix metal or alloy, and the relative proportions of the carbide materials to one another and to the matrix metal or alloy.
Cemented tungsten carbide is classified by grades based on the grain size of WC and the cobalt content and is primarily made in consideration of two factors that influence the lifetime of the tungsten carbide cutting structure: wear resistance and toughness. As a result, cutting elements known in the art are generally formed of cemented tungsten carbide with average grain sizes about less than 7 μm as measured by ASTM E-112 method, cobalt contents in the range of about 6-16% by weight, and hardness in the range of about 86 to 91 Ra.
For a WC/Co system, it is typically observed that the wear resistance, which is related to hardness, increases as the grain size of tungsten carbide or the cobalt content decreases. On the other hand, the fracture toughness increases with larger grains of tungsten carbide and greater percentages of cobalt. Thus, fracture toughness and wear resistance tend to be inversely related: as the grain size or the cobalt content is decreased to improve wear resistance of a specimen, its fracture toughness will decrease, and vice versa.
Due to this inverse relationship between fracture toughness and wear resistance, the grain size of tungsten carbide and cobalt content are selected to obtain desired wear resistance and toughness. For example, a higher cobalt content and larger WC grains are used when a higher toughness is required, whereas a lower cobalt content and smaller WC grain are used when a better wear resistance is desired. The relationship between toughness and wear for carbide composites having varying particle size and cobalt content is shown in FIG. 1.
Accordingly, there exists a continuing need for improvements in the material properties of composite materials used drilling or cutting tool applications.